The generalized feedback shift register pseudorandom number algorithm has several advantages over all other pseudorandom number generators. These advantages are: (1) it produces multidimensional pseudorandom numbers; (2) it has an arbitrarily long period independent of the word size of the computer on which it is implemented; (3) it is faster than other pseudorandom number generators; (4) the “same” floating-point pseudorandom number sequence is obtained on any machine, that is, the high order mantissa bits of each pseudorandom number agree on all machines— examples are given for IBM 360, Sperry-Rand-Univac 1108, Control Data 6000, and Hewlett-Packard 2100 series computers; (5) it can be coded in compiler languages (it is portable); (6) the algorithm is easily implemented in microcode and has been programmed for an Interdata computer.
The image displayed in computed tomography is a scaled representation of attenuation coefficients within the patient's body. A number of authors have presented methods by which additional information (such as electron density, effective atomic number, and extrapolated attenuation coefficients for therapy applications) can be extracted from CT scans carried out at different energies. In the present paper, the dual-energy method described by Rutherford has been used to produce complete images of effective atomic number and electron density of a known phantom (the AAPM phantom) in order to investigate the usefulness of applying this method to current commercial scanners.
A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three-dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed-length water-bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross-sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.
Dielectric measurements and X‐ray diffraction studies were made in the system BaTiO3‐BaHfO3 for polycrystalline specimens containing from 0 to 30 mole % BaHfO3. The maximum in the dielectric constant for each composition in creased as the BaHfO3 concentration was in creased to 16 mole % and then decreased with further BaHfO3 additions. Room‐temperature lattice constants and hysteresis loops were meas ured for all compositions. A partial solid‐state phase diagram is suggested for the solid solution area of the system. The phase diagram and the dielectric behavior of the system up to 16 mole % BaHfO3 are explained on the basis of Devonshire's thermodynamic theory. The ferroelectric‐paraelectric transition in the composition containing 16 mole % BaHfO3 was essentially of second order and occurred between a ferroelectric rhombohedral phase and a paraelectric cubic phase.
A model for calculating mammographic spectra independent of measured data and fitting parameters is presented. This model is based on first principles. Spectra were calculated using various target and filter combinations such as molybdenum/molybdenum, molybdenum/rhodium, rhodium/rhodium, and tungsten/aluminum. Once the spectra were calculated, attenuation curves were calculated and compared to measured attenuation curves. The attenuation curves were calculated and measured using aluminum alloy 1100 or high purity aluminum filtration. Percent differences were computed between the measured and calculated attenuation curves resulting in an average of 5.21% difference for tungsten/aluminum, 2.26% for molybdenum/molybdenum, 3.35% for rhodium/rhodium, and 3.18% for molybdenum/rhodium. Calculated spectra were also compared to measured spectra from the Food and Drug Administration [Fewell and Shuping, Handbook of Mammographic X-ray Spectra (U.S. Government Printing Office, Washington, D.C., 1979)] and a comparison will also be presented.
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